The present invention relates to the manufacture of moulded plastics products and more particularly to the manufacture of plastics products of substantial size and weight. More specifically, but not exclusively, the invention relates to the manufacture of plastics products from post-consumer recycled plastics waste.
Conventional plastics injection moulding techniques traditionally involve the use of high injection pressures. The mould must be capable of withstanding the injection pressures involved and as a result when articles of relatively large size are being produced the cost of the injection equipment and mould is very substantial.
Traditional injection moulding equipment which relies on the use of relatively narrow runners leading into the mould requires the use of virgin plastic or recycled plastic that is substantially free of foreign matter likely to interfere with the injection process. Post-consumer recycled plastic is likely to contain particles of metal glass or stone which for most traditional injection moulding methods must be removed prior to re-use of the recycled plastics. The processing required to ensure that the recycled plastics is substantially free of foreign matter further adds to the cost of processing which may make the cost of using recycled plastics not commercially worthwhile.
The present invention seeks to provide a method and apparatus for relatively low pressure injection moulding of large plastics articles in one shot possibly using post-consumer recycled plastics which may still contain particles of foreign matter.
According to the present invention, there is provided a method of injection moulding a plastics article under relatively low injection pressures, comprising feeding plastics material from an extruder into an accumulator, and displacing the extrudate from the accumulator into a mould via a valve leading into the mould cavity, the valve having a valve piston which at the end of injection closes the mould cavity substantially at the mould surface.
In a preferred embodiment of the invention, during injection of extrudate from the accumulator into the mould cavity the extruder continues in operation to provide further extrudate for use in the same injection cycle.
The extruder may be fed with plastics material composed at least partially of post-consumer recycled plastics, the valve passage being of a size such that it will not be blocked by stones or other particulate foreign matter which might be present within the material. The relatively large cross-sectional area of the valve passage also provides relatively low pressure, but high volumetric flow, of injection into the mould cavity. Advantageously, filling of the mould with the required amount of plastics material is determined by sensing of pressure within the mould cavity.
According to another aspect of the invention, there is provided an injection moulding system for single shot injection moulding of large plastics components, comprising an extruder, and an accumulator fed from the extruder, the accumulator leading into the mould via a mould valve operative when closed to close the mould cavity substantially at the mould surface and also to close an outlet from the accumulator.
In a preferred embodiment of the invention, the accumulator comprises an accumulator cylinder having a piston operative to inject extrudate into the mould, and the mould valve comprises a valve member in the form of a piston within a tubular valve passage terminating in an injection nozzle lying within the mould cavity, the valve piston having an end face which, in the closed condition of the valve, lies substantially at the outlet end of the injection nozzle and seats within the nozzle. Advantageously, an outlet from the accumulator branches into the valve passage, and, in the closed condition of the mould valve, a side wall of the piston closes the outlet at the position at which it branches into the valve passage.
Still further according to the invention, there is provided a system for injection moulding relatively large plastics articles, comprising an extruder, an accumulator coupled to an outlet from the extruder, the accumulator leading into a mould via a mould valve having a passage of relatively large cross-sectional size whereby to effect injection at relatively low injection pressures, and means for sensing a mould-full condition to effect closure of the mould valve by sensing the pressure of injected material at one or more positions within the mould cavity.